The optical elements used in light sources for semiconductor photolithography deteriorate over time, which in turn leads to a decline in light source performance that cannot be remedied while the system is in production use. For example, calcium fluoride (CaF2) is a crystalline material commonly used in deep ultra-violet (DUV) light sources for various applications such as, e.g., windows, beamsplitters, and reflectors. While CaF2 is a robust material for extended use in lithography systems, degradation of CaF2 may occur via any one of several mechanisms. Aside from catastrophic fracture, these mechanisms tend to degrade performance of the CaF2 optic slowly. Initially, such degradation may be relatively inconsequential, but eventually the optical performance of the affected component deteriorates sufficiently that the light source must be taken off line to replace the optic or the module containing the optic.
One particular optical degradation mechanism involves the formation of slip planes. Slip planes occur when thermo-mechanical stresses induce subtle fractures in the CaF2 crystal lattice. Visually, slip planes appear as lines on the optic that are disposed at well-defined angles relative to the orientation of the crystal lattice. Visual inspection is practical only after the damaged optic has been removed from the light source, which can occur only when the light source has been removed from production.
Another optical degradation mechanism is the formation of line damage, one form of which results from the superposition of multiple portions of a beam on an optic that has a coating. The combined energy of these multiple portions may exceed a damage threshold for the coating on the optic. When this occurs it damages the coating in the area of superposition, which creates a distinct line that can diffract light. The line damage is a problem because it can cause distortions in the beam profile that reduce module lifetime. It would be advantageous to be able to detect the existence and extent of line damage.
Yet another aspect optical degradation mechanism is the presence of point-like defects due to particles on an optic at some location within the system. Such particles typically do not have an undue adverse effect on optical performance because they are so few in number, but they can affect image processing. It would be advantageous to be able to identify the presence, location, and nominal size of point-like defects.
An additional damage mechanism that degrades optical performance is the onset and growth of dendritic formations. These dendrites resemble a white fuzz, and they are commonly referred to as such. The presence of dendrites indicates a coating failure, which leads to localized heating of the optic in the vicinity of the dendrites. The heating exacerbates coating failure, so the affected region of the optic expands, which further exacerbates the localized heating and causes thermal lensing. The optical performance of the laser then degrades, and eventually the affected optics must be replaced. It would be advantageous to be able to detect the onset and progression of dendrite formation.
A common aspect of the challenges associated with these damage mechanisms is a need for a way to detect and identify low-level signals associated with these defects, and for attributing these signals to specific defect types. A further need exists for ways of monitoring the evolution of these defects that allows prediction of how much time remains before light source optical performance decreases sufficiently that an optic or module must be replaced.
Eventually, the light source must be taken out of production use so degraded modules and optics can be replaced. This obviously leads to downtime for the scanner. It is desirable that downtime be minimized to the fullest extent possible. While planning maintenance events can reduce the adverse impact of lost production time, all downtime comes at a significant cost to chip makers. The total amount of time the light source is out of production is sometimes referred to as the “green-to-green” time.
During maintenance, once any required replacement modules and optics have been installed, the light source must be re-aligned. As mentioned, the light sources can be embodied as a set of modules. In such cases, alignment typically involves adjusting positioners that tip and/or tilt individual optical elements within the modules in some cases, and entire modules in other cases. Once aligned, the light source can be used for significant amount of time without additional adjustment. The amount of time required to realign the optics and modules adds disadvantageously to the green-to-green time. Alignment may be carried out by the use of mechanical alignment actuators but this can be unduly time consuming.
One part of the alignment process may be carried out to compensate for chamber resonances. Laser radiation for semiconductor photolithography is typically supplied as a series of pulses at a specified repetition rate. Chamber resonances may occur at some repetition rates and cause sharp increases in performance metrics, (e.g. pointing and divergence) near the resonant frequencies, with low valleys or floors at frequencies adjacent to the resonance. The presence of resonances per se may be acceptable provided all the data points remain within specification, although additional time and effort may be required during alignment to keep the performance metrics within specification. In addition, a resonance caused peak-to-valley differences in performance metrics may create technical challenges for scanner design and control.
Measures to minimize the total amount of green-to-green time include both reducing the number of maintenance events and reducing the length of the maintenance events when they do occur. The number of maintenance events can be reduced by extending the operational life of the modules and optics and by having a reliable way of determining the amount of optic damage in situ. The length of the maintenance events can be reduced by reducing the amount of time needed to align replacement modules and optics.